Odd spot on Dock Bar. What is it and can it be removed?

I have an odd white spot on my dock bar and would like to remove it.

It appears only on this Mac Mini, 2018, all up to date with the Mac OS Monterey 12.3.1.

It cannot be clicked on, but is movable by re-arranging the icons either side.

It appears to be benign. Except that it is annoying, distracting and looks out of place.

It does not appear on my Macbook Air's (2020 M1) Dock, which has the same same OS and apps installed as this Mac Mini.

Could it be a remnant of a removed app? Any ideas on how to remove it?

Seen here sitting between Finder and Launchpad.